Abstract. Biological communities populating the Mediterranean Sea, which is situated
at the northern boundary of the subtropics, are often claimed to be
particularly affected by global warming. This is indicated, for instance, by
the introduction of (sub)tropical species of fish or invertebrates that can
displace local species. This raises the question of whether microbial
communities are similarly affected, especially in the Levantine basin where
sea surface temperatures have significantly risen over the last 25 years
(0.50 ± 0.11 °C in average per decade, P < 0.01). In this paper,
the genetic diversity of the two most abundant members of the phytoplankton
community, the picocyanobacteria Prochlorococcus and Synechococcus, was examined during two cruises
through both eastern and western Mediterranean Sea basins held in September
1999 (PROSOPE cruise) and in June–July 2008 (BOUM cruise). Diversity was
studied using dot blot hybridization with clade-specific 16S rRNA
oligonucleotide probes and/or clone libraries of the 16S-23S ribosomal DNA
Internal Transcribed Spacer (ITS) region, with a focus on the abundance of
clades that may constitute bioindicators of warm waters. During both
cruises, the dominant Prochlorococcus clade in the upper mixed layer at all stations was
HLI, a clade typical of temperate waters, whereas the HLII clade, the
dominant group in (sub)tropical waters, was only present at very low
concentrations. The Synechococcus community was dominated by clades I, III and IV in the
northwestern waters of the Gulf of Lions and by clade III and groups
genetically related to clades WPC1 and VI in the rest of the Mediterranean
Sea. In contrast, only a few sequences of clade II, a group typical of warm
waters, were observed. These data indicate that local cyanobacterial
populations have not yet been displaced by their (sub)tropical counterparts.